Semiconductor devices and parts are extremely hindered by dirt, dust and other contamination. Such devices and parts are therefore produced in clean rooms, while the garments, gloves and other operating utensils used must all be clean.
Clean rooms are controlled so as to maintain a prescribed degree of cleanliness. When clean parts, utensils, garments and the like for production of semiconductors are placed in bags to be brought into clean rooms, it is essential to use clean bags which are, naturally, free of dust inside, but which are also free of dust on both the inner and outer surfaces, since dust clinging to the outer surface will contaminate the clean room.
Known methods for cleaning of bags include washing in ultrapure water, and cleaning of films or sheets prior to manufacturing of the bags inside clean rooms. Another method involves coating both the front and back sides of an inner film or sheet for wrapping with a peelable outer film or sheet to prevent contamination of the inner film or sheet, and peeling off the outer film or sheet at the time of use and using the inner film or sheet to wrap a given article to be wrapped (Japanese Unexamined Patent Publication No. 6-285944).
In the method involving coating of the film or sheet with an outer layer, the bag must be wrapped after peeling off of each outer layer, while waste of the outer layer is also not economical.
Although cleaning of films and sheets by washing with ultrapure water is commonly known, almost no specific methods have been published. Suitable ultrapure water must be prepared for high cleaning of films and sheets, and the washing and drying methods must be specially modified. As a result, it has not been possible to achieve sufficient cleanliness of conventional plastic bags even by washing with ultrapure water.
In general, the cleanliness of films, sheets and bags is expressed relative to the clean room, such as in terms of causing no contamination of the clean room even when, for example, a class 10 clean room (according to Fed. Std. 209D. Same hereunder) is used, and the cleanliness of the sheets or bags themselves is never expressed quantitatively. Therefore, the expression of cleanliness is imprecise and subjectively influenced.
Measurement of fine particles on the surfaces of films or sheets can be accomplished, for example, with an optical microscope, but generally only sizes of 5 .mu.m or greater may be detected, while fine particles with extremely small sizes of under 1 .mu.m are almost impossible to detect. In addition, the very narrow visual field of the microscope does not allow easy measurement of fine particles in sections having certain areas. Detection of fine particles is also difficult in sections where the surface is bent or irregular.
The present invention provides a highly cleaned plastic film or sheet and a process for its production, and more preferably it provides a plastic film or sheet with high cleanliness through the successive process from molding of the film or sheet until its cleaning and formation into a bag, and particularly a process for manufacturing plastic bags.